This invention is in the field of bridge rectifier circuits, and relates more particularly to bridge rectifier circuits having active switching elements.
Typical prior-art bridge rectifier circuits use four diodes in a bridge configuration having two AC input terminals and two DC output terminals, and serve to convert an AC input voltage into a DC output voltage. Perhaps the most common use of such bridge rectifier circuits is to convert an AC input voltage derived a power line into a DC voltage which is subsequently filtered and used to power electronic circuits.
Although the simple four-diode bridge rectifier circuit performs its intended function adequately, it does suffer from a number of drawbacks, particularly in high-current applications. Because of the inherent forward voltage drop (typically 0.7 volts or higher) across each diode, significant amounts of power can be dissipated in the bridge, particularly when large currents are involved. This unwanted power dissipation requires the use of larger components and generates unwanted heat, drawbacks that are particularly bothersome in integrated circuit applications.
To overcome these drawbacks, prior-art bridge rectifier circuits were developed in which MOS transistors were used to replace at least two of the diodes in a conventional bridge rectifier circuit. The advantage of using transistors instead of diodes in this application is that an MOS transistor, when turned on, provides a substantially lower voltage drop (in the order of 0.2-0.3 volts) thus reducing power dissipation and heat generation as compared to diode rectifiers.
When MOS transistors are used as rectifying elements, however, additional circuitry must be provided to turn the transistors on or off at the appropriate times. In the prior art, this function is accomplished by connecting the gates of the MOS transistors (either directly or through a network of passive components) to the AC input voltage terminals, thus providing a simple and reasonably effective source of control voltage to appropriately switch the MOS transistors. However, since this technique results in a sinusoidal control voltage, with a slow transition from low to high voltage, the MOS transistors will not be efficiently switched, and will be only partially on during a significant portion of the duty cycle. This results in a relatively high on resistance during this portion of the duty cycle, thus resulting in higher power dissipation and substantially reducing the advantage achieved by substituting MOS transistors for diodes as rectifying elements. Furthermore, the variable resistance of the MOS transistors may result in undesirable "glitches" in the output waveform when the circuits are required to carry large currents.
Accordingly, it would be desirable to have a bridge rectifier circuit in which voltage drop, power dissipation, heat generation and output waveform "glitches" are all minimized.